Solid state linear scanning arrays are known for the electronic capture of image data from documents or other objects. A linear array inherently requires relative movement with respect to the image source to progressively expose line images which together constitute an image area. Solid state linear arrays currently operate by developing, point-by-point in individual cells of the array, electrical charge proportional to the light exposed to corresponding individual sensing devices. The speed at which images can be processed is limited by the inherent light sensitivity of the sensing devices, as well as the strength of the available light source.
One known technique for increasing the sensitivity of such a scanner is known as time delay and integration (TDI) which is described, for example, in U.S. Pat. No. 3,833,762. A TDI array provides a series of linear arrays whose charge developing cells are coupled in the direction of relative movement of the image, such that the same line image progressively is exposed to succeeding ones of the linear arrays. The coupling of the developed arrays enables charge developed by the cells of each array to be transferred to a subsequent cell in the series in a timed relation to the progress of the line image along the series of arrays. This arrangement effectively allows a longer total exposure time of a given line image to sensing devices. In fact, the exposure time can be increased as desired by simply increasing the number of cells in the series to which the line image will be exposed.
To operate such a TDI array, certain control signals must be applied to the cells to cause the development and transfer of electrical charge at proper times. These signals must be applied at a rate that is related to the speed of progress of the line image relative to the scanner array. Normally, the conductors for applying these signals are made part of the solid state device by the use of semi-transparent conductive material such as polysilicon. The transparency of the material is required in order that the control member not interfere with the reception of light by the underlying photosensitive surface. The speed and accuracy with which the control signals can be applied are limited by the relative low-conductivity of the polysilicon, which becomes particularly significant in a TDI array due to the long distances over which the signals must be conducted. The speed of application of the control signals directly limits the permitted speed of the line image relative to the array. The accuracy of the control signal is especially significant in that any error in the transfer of charge causes an accumulated error which can degrade, if not completely destroy, the image being progressively captured.